1. Field of the Invention
This invention relates to relay circuits and, more particularly, to methods and apparatus for compensating for temperature changes affecting the overload trip level in such circuits.
2. History of the Prior Art
A great variety of solid state relays circuits have been devised which use a power semiconductor as the output switching device and use a silicon controlled rectifier or similar latching circuit to sense a current overload or short circuit condition. For example, U.S. Pat. No. 4,581,540 discloses such a circuit.
Although the circuit disclosed in the prior art patent application accomplishes its purpose well to limit overload current in normal operating conditions, the circuit does have a relatively high trip voltage of approximately 450 millivolts at room temperature of 25 degrees C., and that trip voltage is sensitive to changes in temperature in the operating environment. For example, when the circuit is used in environments in which the temperature varies from -55 degrees C. to 115 degrees C. (a relatively normal range for aircraft operation, for example), the trigger level at which the current protection trips the cutoff of the switching device varies by approximately 400% from 150 to 600 millivolts.
Although the objectives can be met by using operational amplifiers and comparators circuitry, such arrangements require the use of additional power supplies, eliminate the ability to provide optical isolation of the input, and often incur large increases in input current. These changes increase the cost, complexity, and size of the circuitry while decreasing its reliability.